CDK2, also known as cyclin-depentent kinase 2, is a member of the Ser/Thr protein kinase family. It is highly similar to the gene products of S. cerevisiae cdc28, and S. pombe cdc2. It is a catalytic subunit of the cyclindependent protein kinase complex, whose activity is restricted to the G1-S phase, and is essential for cell cycle G1/S phase transition.